I found out this cheat in a guide book of the game. You may doesn`t believe it, but it does REALLY WORK!

1: First, you must take the Elite 4 two times (be sure you use your starting pokemon fully evolved).

2: Talk to prof. Oak, and he will check your pokedex. You now go to the mart in veridian and by some pokeballs.

3: Catch a caterpie in lvl. 3 in the viridian forest. You must now train it to a very high lvl. to beat the elite 4 once again with your butterfree, your starter, one of the legendary birds, and three pokemon you may choose you self.

4: You must now go into the cave you can catch Mewtwo (be sure to have already caugth or killed him). Then you go to the same spot where you found him, go out on the toilet (only to draw out some time, cause you have to wait in 5 minutes), and then you take of your hat, and it will appear a weedle in level 2!